golf mk3 tdi 1996 glow light flashing.

[samgti]

Hi there,my golf's glow light starts flashing 20 minutes after start up.
Otherwise the car operates normaly.
when glow light starts to flash if i restart the car it will disappier for a cycle of 20 min.
Has anyone had this problem befor?I was thinking maybe the glows need replacing,but i dont know if its been done before.
It has done 275"000 kms and has been a good reliable runaround since i've 75000 kms on it.
it's stock standardand only has a 3"down pipe stright threw no muffler,with a performance tunning box.
I've taken that out for a day but the light still flashes
any ideas whay this is hapenning?
thank you all.